Question

Why sulphur trioxide formed in absorption tower in contact process is not absorbed directly in water

Answer 1

SO3 dissolution in water is extremely exothermic. This forces the sulfuric acid to form a mist of tiny droplets. These are highly corrosive which can even attack and corrode the lead pipelines and linings which cover the tower in contact process. Also, this mist does not condense so easily. So, sulfur trioxide is dissolved in concentrated sulfuric acid to produce oleum/ pyrosulfuric acid (H2S2O7). equialent to sulfuric acid content of about 110% )which can be diluted with a required amount of water (in a much less exothermic pathway) to yield con (98%) sulfuric acid.